In The Elder Scrolls V: Skyrim for the PC, PS3, and Xbox 360, dragons can be a little evasive at times and not want to land in order to fight them. In some instances, this can be a bug, however I do remember reading interviews with the Bethesda staff saying that some dragons won’t attack until provoked.
First off, make sure you have the latest patch to the game. The first patch to the game listed fixing an issue with dragons not attacking.
If that doesn’t help you out, one trick I’ve used is to hop on my horse and ride around in sight of the dragon. For some reason, dragons like to eat horses or something, because I’ve used this trick on multiple occasions.
Another method is to use the Dragonrend shout, which forces a nearby dragon to land. You get the Dragonrend shout during the primary storyline however, and it’s a little way into the story, so it may be a while until you get it unless you’re just plowing through the story.
If you’re curious when you get Dragonrend, it’s a little while after you visit Alduin’s Wall, so look up that quest if you want to get your overall progress in the main story line.